68 
Lee . — The Morphology of Leaf-fall. 
The living vascular elements above undergo complete chemical change, 
the wood and bast parenchyma and the companion cells becoming ligno- 
suberized and the sieve tubes submitting to lignification. 
During the second year the cambium of the Protective-layer resumes 
its activity, and usually gives rise to many layers of phelloderm. Some- 
times isolated cells in the periderm have been observed to develop a cuticle 
similar to that just described ; and by the end of the second year, a second 
cuticle, identical with the first and situated a few cells below it, has been 
produced. This is generally well shown in cases where the axillary bud 
has become aborted (Text-fig. 11 ). The conditions of the leaf-scar are 
then reproduced, and the cuticular layer is continued beneath that of the 
leaf-scar and joins the internal cuticle of the stem periderm, which has by 
this time been produced. In all cases the corresponding cells within the 
leaf-trace also develop cuticular walls. Additional periderm and internal 
cuticles are formed in subsequent years. 
This striking formation of internal cuticle beneath the leaf-scar is not 
confined to 5. Caprea, but it is also found in all the species of Salix 
examined, i. e. 6*. laurina , hippophae folia, purptirea , and var. S char fen- 
bur gensis, S. alba , var. caerulea , .S'. cordata , Smithiana , var. acuminata , 
5. cuspida , undulata , rubra , daphnoides , and Babylonica , var. Salmoni , and 
S. incana. Many variations are apparent in the time of formation and 
general appearance of the internal cuticle, and without entering into details 
we may just point out one or two of these. The time of appearance relative 
to the formation of the secondary cuticle of the stem periderm varies very 
much ; in S. Smithiana , var. acuminata , and .S', daphnoides , the time of 
formation agrees with that described for .S'. Caprea , for here the secondary 
cuticle affords a protection for the scar before the appearance of the internal 
cuticle of the stem. This is very different from what as a rule occurs in 
the remaining species, in which the scar cuticle has a belated appearance 
and often seems to be formed as a result of the continuation of the internal 
cuticle of the stem beneath the scar. It is possibly right to conclude that 
the degree of dissociation which exists in some species between the forma- 
tion of the secondary cuticle of the scar and that of the stem is due to the 
difference in the time of formation of the periderm in the stem, for it is only 
in cases where the formation of the stem periderm is late that the internal 
scar cuticle has a separate origin. 
Another difference to be noted is the different degree of thickness 
attained by the internal cuticle. The primary cuticle is invariably thick 
and conspicuous, and in most cases the internal cuticle is very similar. 
But in S. incana it is so thin that it can scarcely be recognized even the 
second year. Correlated with this, the periderm produced beneath the 
scar in this species is much more abundant than is usually the case. 
